FIELD OF THE INVENTION
This invention relates to a hinge intended primarily, although not exclusively, for a roof window of the kind wherein a glazed sash is pivotally openable relative to a fixed peripheral frame, about an axis extending transversely of the fixed frame in the centre region thereof. The usual mode of installation of such a window in a conventional sloping roof is that the pivot axis of the opening sash lies horizontally, mid-way between upper and lower edges of the sash.
In such a roof window, the disposition of the pivot axis of the sash mid-way between the upper and lower edges thereof is to enable the sash to be pivoted through an angle approaching 180 degrees from the closed position, to enable cleaning from inside the building of the glass surface which is the external surface when the window is closed. It will be appreciated that access for cleaning the exterior of a roof window usually would be somewhat difficult if the window were not capable of so opening.
Several other design constraints exist in respect of roof windows. It is usual to provide a window with covering elements, which may be in the form of sheet metal components commonly termed cassettes, to provide weather protection for the joint faces between the opening sash and the fixed frame. Weather seals, of course, are also provided at such joint faces. In any event, the hinges used must not interfere with the fitting of the cassettes. Further, the hinges should be capable of affording a, preferably adjustable, frictional resistance to pivoting of the sash so that it will hold a desired position between its fully open and fully closed positions, against possible unbalance of the sash and external effects such as wind. Desirably also at least one positive stop is provided for the sash to maintain a fully opened (reversed) position for cleaning purposes and a slightly open vent position.